Galactic cannibal Astronomers have witnessed the rare event of a black hole waking up from a decades-long slumber to feed on a planet-sized object in a galaxy 47 million light years away.

The observation, made using the European Space Agency's Integral satellite project, is outlined today in a paper published in the Astronomy and Astrophysics journal.

"The observation was completely unexpected, from a galaxy that has been quiet for at least 20-30 years," lead author Marek Nikolajuk of Poland's University of Bialystok says.

It also comes as astronomers wait on a similar feeding event, albeit on a gas cloud, that is expected to soon happen at the black hole at the centre of our own Milky Way Galaxy.

The observation revealed a black hole that had been slumbering for years chomping on a giant, low-mass object that had come too close.

Scientists from the University of Geneva analysing the data collected by Integral, spotted a light flare coming from a black hole in the centre of the NGC 4845 galaxy, which has a mass more than 300,000 greater than the Sun and had been dormant for more than 30 years.

Matter-sucking black holes normally lurk dormant and undetected at the centre of galaxies, but can occasionally be tracked by the scraps left over from their stellar feasts.

Playing with food

This black hole had woken up and absorbed an object with a mass 15 times that of our own Jupiter.

The paper also shows this black hole likes to play with its food: the way the emission brightened and decayed shows there was a delay of two to three months between the object being disrupted and the heating of the debris in the vicinity of the black hole.

This is the first time where we have seen the disruption of a substellar object by a black hole," says co-author Roland Walter of the Observatory of Geneva, Switzerland.

"We estimate that only its external layers were eaten by the black hole, amounting to about 10% of the object's total mass, and that a denser core has been left orbiting the black hole."

In a statement, the European Space Agency says the object was either a giant planet or a brown dwarf - stars that lacks sufficient mass to sustain a thermonuclear fusion of hydrogen and helium which makes other stars, like our sun, shine brightly.

Astronomers estimate there as many errant planets in the Universe as there are stars - meaning plenty for lunch options for black holes.